This invention relates to a method of installing a rivet in a plurality of workpieces, and to the resulting riveted joint, as well as the rivet itself.
One rivet type fastener commonly used in aircraft constructions includes a shank with a manufactured head on one end and a tail on the other end. In use, the tail end of the shank is inserted through aligned holes of two or more workpieces with the rivet head engaging the outer face of one of the workpieces and with the tail extending beyond the outer faces of the other workpiece. The tail is then deformed by means of an axial force, compressing the rivet axially and upsetting the tail material outwardly to form an upset head which is larger in diameter than the hole through the workpieces, so that the two workpieces are fastened together. One widely used rivet of this general type is of the bi-metallic variety, comprising a shank made of a strong material which is high in shear strength and a tail made of a more ductile material which is easier to deform than the shank.
All types of fasteners having a tail to be upset are often installed by squeezing, wherein the ductile tail is compressed until the upset head is formed therefrom. A general problem associated with rivet installation of this type is that when the squeezing force used to form the upset head is released, the column of the rivet shank "springs back" or lengthens a certain distance due to elastic memory. Although the material of the workpiece being fastened also springs back, most of the materials in common use do not spring back as much as the rivet shank, with the result that a small gap is created between portions of the upset head and the workpiece after the installation is complete. This gap is undesirable in that it provides a location for moisture to collect, thereby promoting corrosion of the workpiece. Moreover, this gap is unacceptable for applications where the workpiece and fastener are to be subjected to high fatigue loads.
In aircraft structures, particularly those involving tension fatigue loading of the fastener, it is desirable that the gap between the upset head and the workpiece be zero. This is in part because, with the gap eliminated, the upset head will be flush with the underlying workpiece, thereby providing an improved aerodynamic profile for the aircraft structure. Ideally, the underside of the upset head should exert a compression force against the workpiece after the installation. When such a loading is achieved, the fastener is said to exert a residual tension force against the workpiece after installation. This loading is often referred to as a "preload" in the joint. The advantages of preload are especially desirable in aircraft construction because preload provides for a higher fatigue life of the joint and provides excellent protection against corrosion because it becomes difficult for a corroding substance to infiltrate inner surfaces of the joint.
A general problem in this area is an inability to obtain a predictable, measured preload in a fastened or riveted joint. For example, preload is obtainable with conventional two piece fasteners, such as a nut and bolt, but it is very difficult to quantify the amount of preload achieved because of other factors present such as friction, type of materials, etc. Moreover, two piece fasteners present serious feeding problems when automatic or robotic installation is attempted. Thus, although a difficult to quantify preload is achievable with two piece fasteners, use of such fasteners is still not as preferred as one piece fasteners in the aircraft industry because automated fastener installation of one piece fasteners is the preferred mode in aircraft construction due to the lower costs and improved installation uniformity associated therewith. One prior practice which attempts to address the problem of providing preload in a riveted joint involves the use of hot rivets which, after being upset, contract upon cooling and produce the desired preload in the joint. However, this hot rivet approach is not a practical method for obtaining preload in aircraft structures because of the higher costs and complexities associated therewith.
A one-piece fastener is particularly desirable as opposed to a two-piece fastener in that it is easy to feed and install using automatic equipment. A predominant type of one-piece fastener in use is the afore-mentioned bi-metallic rivet having a strong shank and a ductile tail. However, an inability to provide a preload had previously been encountered with the use of bimetallic rivets. This drawback was addressed in Applicant's prior U.S. Pat. Nos. 4,688,317 and 4,904,137, incorporated herein by reference. Unfortunately, the teachings of Applicant's above-noted prior patents apply best when the rivet to be installed comprises a manufactured head (such as 55 Ti 45 Cb titanium alloy) which is much harder than the shank material (i.e. a bi-metallic rivet). Thus, a method of obtaining preload during the installation of solid ductile rivets, rather than bimetalic rivets, is an area which has yet to be addressed in an ideal manner. The widespread use of standard solid ductile rivets in the aerospace industry today requires that an effective method of obtaining preload in solid, one-piece ductile rivet installation be achieved.
There exists therefore, a significant need for a method of installing a solid one-piece ductile rivet or shear pin fastener in a manner which can provide a significant axial preload. Moreover, such a method is needed which allows for automated rivet installation using machines, and which enables a predictable, quantifiable preload to be obtained. Further, such a method is needed which is compatible for use with universal head rivets as well as with flush head rivets. The present invention fulfills these needs and provides further related advantages.